The air system does not lower when you get out, it levels the car and stays level, but is you put weight in trunk and people sitting in the back seat and the car lifts to level out with the weight and then you remove all the weight and rear end is to high up then there is a problem.
These bags do not lift that high, if you hear the solenoids make sure it is not lower , measure.Also, the release of the air is very slow, make sure it is not lower, if you hear the solenoids open the car may be lowering very slow. check the height.If you are sure its not lowering then you may be correct in saying the level sensor may be bad. the only other part would be the solenoids but i would start with the level sensor as the air is going into the bags.
I quick test to check solenoids at bags is to start car and go rear the rear and locate the sensor, remove the arm from the control arm and bring it down so computer thinks car is to high and see if air is released. caution don"t stay under the car as it will lower down slowly. if it starts to lower then turn key off and reconnect lever and replace sensor.there is a company we use that is real good at this air bag system and prices are good with quality parts.http://www.arnottindustries.com/part_LINCOLN_Air_Suspension_Parts_yid2.html